An LED chip that uses a nitride semiconductor (InXGaXAl1-X-YN, 0≦X≦Y≦1), a semiconductor light emitting element capable of emitting blue light with high brightness has recently been developed. Light emitting devices based on nitride semiconductor have such advantages as high output power than those of light emitting devices that use GaAs, AlInGaP or the like to emit light within a range from red to yellowish green and less color shift caused by temperature change, but has such a drawback that it is difficult to obtain a high output power in a wavelength region including the wavelength of green light and longer. On the other hand, the applicant of the present invention have developed a light emitting diode that is capable of emitting white light by disposing YAG:Ce fluorescent material, a fluorescent material capable of absorbing a part of blue light emitted by the LED chip described above thereby emitting yellow light, on the LED chip and have applied for patent (International Publication No. WO98/5078).
The light emitting diode is, despite the relatively simple constitution of single-chip two-terminal configuration, capable of synthesizing light from the LED chip that is electrically connected to a lead electrode and light from a fluorescent material such as YAG:Ce included in a light transmitting resin that covers the LED chip, and emitting the synthesized white light through a convex lens.
The light emitting diode allows it to modify the synthesized light emitted by the light emitting diode from bluish white to a desired color such as yellowish white by adjusting the content of the fluorescent material. It may also be possible to emit yellow or red light, for example, by adding a pigment.
Meanwhile diversification of the applications for the light emitting diode has generated the needs for a light emitting diode that can emit light with higher luminance.
In recent years, chip light emitting diodes are widely used as the light source for illuminating switch, full-color display, back light for liquid crystal display and the like. The chip light emitting diode is made by electrically connecting a light emitting element in a recess of a package and covering the light emitting element with a light transmitting resin so as to seal the chip.
On the other hand, the rapid advancement in the optical semiconductor technology has been causing a great increase in the output power of the optical semiconductor devices and remarkable decrease in the wavelength of the emitted light. A light emitting diode made of a nitride semiconductor, for example, can emit light with peak intensity at any wavelength within a range from about 350 nm to 650 nm depending on the elements that constitute the light emitting layer. It has also be made possible to emit light with a high output power of 5 mW or over in visible light region of wavelengths not longer than 550 nm (including near ultraviolet and blue-green light) by using multiple quantum well structure in the light emitting layer of a nitride semiconductor. Such a high output power gives rise to a new problem. That is, for a optical semiconductor device that can emit or receive light of such a high energy, it is important to prevent the molding resin from being deteriorated by the light and reduce the stress generated by heat between the molding resin and the optical semiconductor chip.
Japanese Unexamined Patent Publication No. 2000-196151 discloses a molding resin containing, as a main component, an alicyclic epoxy resin instead of bisphenol epoxy resin. An epoxy resin composition obtained by curing the alicyclic epoxy resin, as a main component, with an acid anhydride contains substantially no carbon-carbon double bond, that causes light deterioration, in a main skeleton thereof. As a result, such an epoxy resin composition is less susceptible to deterioration of the molding resin even after being irradiated with light for a long period of time and has relatively good pliability. Thus the semiconductor chip is less likely to be damaged by the thermal stress.
However, epoxy resin cured by acid anhydride cannot be used in a surface mounted device (SMD) that comprises a semiconductor chip mounted directly on the substrate surface, since such a device requires it to form a thin film of the molding resin. That is, the surface mounted device requires a thin film of the molding resin 1 mm or less in thickness formed thereon, the mixture liquid of epoxy resin that is applied has a larger area exposed to the atmosphere. However, since the acid anhydride curing agent has high volatility and high hydroscopicity and takes a relatively longer time, from 5 to 20 hours, to cure, the acid anhydride curing agent absorbs moisture and evaporates during curing, thus making it difficult for the epoxy resin to properly cure. The epoxy resin that has cured unsatisfactorily cannot demonstrate the proper function of resin with the light and heat resistance significantly deteriorated.
For the reason described above, a cation curing agent such as aromatic sulfonium salt is commonly used instead of the acid anhydride curing agent for such applications that require thin film such as surface mounted device. The cation curing agent allows for satisfactory curing even when a mixture liquid thereof with epoxy resin is applied in a thin film, because of low volatility.
However, since the cation curing agent has an intrinsic tendency to absorb blue light and light having shorter wavelengths, epoxy resin that has been cured by the cation curing agent is liable to discoloration, namely to become yellowish due to absorption of short wavelength light. As a result, it has been difficult to use epoxy resin that is cured by the cation curing agent in a optical semiconductor device that emits or receives blue light and shorter wavelengths. Also because the curing reaction proceeds almost only by the ring-opening reaction of epoxy groups, the epoxy resin composition thus obtained has a three-dimensional network wherein ether bonds are arranged relatively orderly and has lower pliability. Thus there has been such a problem, when epoxy resin that is cured by means of a cation curing agent is used in a optical semiconductor device, that a large stress is generated between the optical semiconductor chip and the molding resin during heating and cooling of the optical semiconductor device, eventually leading to cracks generated in the optical semiconductor chip and wire breakage.
To improve the pliability of epoxy resin composition cured with the cation curing agent, the epoxy resin may be mixed with a low-molecular weight reactive diluent such as monoglycidyl ether, polyglycol glycidyl ether, tertiary carboxyl acid monoglycidyl ether. However, since such a reactive diluent impedes curing of the epoxy resin, it becomes necessary to increase the amount of the cation curing agent added, which aggravates the problem of yellowing of the epoxy resin composition.